The history of electrical energy storage devices, especially capacitors and batteries, has involved attempts to reduce the package size while increasing the electrical energy storage capacity. Recent advances in battery and capacitor design have increased the life, the efficiency, and the energy density. Although many of the devices embracing these advances have filled a need, there continues to be a requirement for efficient, high-powered electrical energy storage devices which can withstand the rigors of continuous use.
The need to store greater amounts of energy in ever smaller packages continues to drive new research into complex and esoteric ways of making storage devices. The result is that many batteries and capacitors have become expensive and complicated and also require sophisticated charging schemes and fabrication methods.
Electrochemical capacitors (EC) are similar to battery cells, in that they store electrical energy but unlike batteries, they rely on charge separation at the electrode/electrolyte interfaces to store this energy. The total energy stored by the EC is, for this reason, relatively small compared to conventional batteries. However, the power output of ECs is much higher than batteries. ECs are, therefore, more useful in applications that require high power in short bursts, whereas conventional batteries are more suitable for use in applications that require moderate power at sustained levels. Because battery cells store electrical energy in the form of chemical energy, a battery is capable of delivering a relatively constant and sustained level of voltage over a period of time, whereas, a capacitor voltage continually decreases as a function of time during discharge.
Power packages that combine one or more batteries and one or more capacitors are well known in the art and these devices attempt to provide the best of both worlds. For example, one well-known application is a photo flash. In this situation, the flash bulb or strobe requires an extremely high burst of energy to fire the strobe. Battery cells are incapable of providing this high intensity energy burst, but a capacitor is very well suited. The photo flash package uses a battery cell to charge the capacitor, and the capacitor is then discharged to provide the instantaneous burst of power. After the flash is over, the battery slowly recharges the capacitor, readying it for the next burst. Another type of application is in portable, hand-held radio transmitter/receivers. While the receiver is in standby mode, it requires a relatively constant but low level of voltage. However, during transmission, the receiver requires a burst of power to transmit the signal. Present day devices attempt to provide this high level of power using battery cells, because a capacitor by itself cannot provide the stable voltage required to sustain the device operation. In both these applications, conventional technology requires that the power package be a mechanical combination of a conventional capacitor and conventional battery cells. Clearly, a need exists for an electrical energy storage device that combines the desirable features of electrochemical batteries and capacitors, and can store large amounts of energy in a smaller package, and be manufactured at a reasonable cost.